The present invention relates to a sensing apparatus for detecting the tensile forces experienced in a strand of flexible binding material while being wrapped and tied around a bale of crop material such as hay or straw, for example.
In a conventional rectangular baler, as exemplified in U.S. Pat. No. 3,416,824, hay, straw or similar crop material that has been previously cut, windrowed or swathed, is picked up from the ground by a pick-up unit and fed in successive batches or charges into an elongated bale chamber in timed sequence with a reciprocating plunger. The plunger compresses the material into bales and, at the same time, gradually advances the bales towards the outlet of the bale chamber. As the bales reach a predetermined length as determined by a metering device, a knotter device is actuated which wraps cord, twine or other flexible binding material around the bale and secures the ends of the binding material together.
In a typical rectangular baler the knotter device, commonly comprising two or more knotters, is mounted on the bale chamber above a slot therein, each knotter comprising a twine holder from which twine extends to encircle a bale. During the baling operation, a leading strand of twine is held by the twine holder and extends forwardly across a knot tying member, known in the art as a billhook, and a twine guiding member and then in front of the bale being formed. The twine guiding member supports the strand so that it does not bear forcefully against the billhook. A needle is involved in completing the encirclement of twine around the bale and, when advancing, the needle lays a trailing strand across the twine guiding member, the billhook and the twine holder. To initiate the formation of a knot by the billhook in the leading and trailing strands of twine, a twine finger captures the strands and positively positions them against a heel of the billhook.
On completion of the operation of the knotter, the twine finger returns to its initial position. The tied knot is mechanically stripped from the billhook by moving the twine guiding member thereacross; the member normally embodying a knife operable to cut the twine from the twine supply so that the tied bale is complete in itself. Finally, evacuation of the completed bale from the bale chamber is effected by starting the formation of a new bale in front of the completed one thereby forcing the latter rearwardly out of the baler.
The present day balers are expected to meet high demands regarding capacity of the baler, as well as density of the bales and reliability of operation. Under certain tough operating conditions, e.g. when baling damp silage crop at a considerable density, the tying process may intermittently fail. Failure occurs either because of breakage of the binding material or because the binding material is pulled from the tying mechanism. Both types of failure usually occur within a few plunger strokes after a tying cycle. It is during this interval that the leading strand of the binding material encounters the greatest resistance to sliding between a completed bale and a new bale being formed as both are pushed toward the exit opening in the bale case. Also, at this stage of the bale formation, only little crop material is present between the plunger and the strand of twine at the leading end of the bale being formed, meaning that the high acceleration forces of the plunger during its compression stroke are not cushioned by the crop material but are almost integrally transmitted to the binding material which thereby is subjected to elevated tensions, sometimes leading, as already mentioned, to the binding material being pulled out from the twine holder or to breakage of the binding material in the vicinity either of the twine holder or the twine guiding member. These areas have been found to be the most critical for twine breakage as the binding material in these regions eventually might be slightly damaged by clasping the twine in the twine holder or by pulling the twine around the twine guiding member, whereby the tensile strength of the twine may be reduced below a level sufficient to resist tensions experienced therein during bale formation.
In current large rectangular balers, bales commonly are wrapped with synthetic twine, made of polypropylene and having a specific length between 130 and 150 m/kg with a corresponding tensile strength between respectively 2900N and 2500N. Using twine with a specific length below 130 m/kg, thus having a larger tensile strength, theoretically could lessen twine failure occurrence. However, such a twine is not currently available in the field of agricultural application and moreover, even if it were, the use thereof could not be recommended as this would require all components of a knotter mechanism to be redesigned and redimensioned completely, which would represent an enormous and expensive job having the complexity of a knotter mechanism in mind.
To at least alleviate the inherent problems of twine failure, the arrangement disclosed in DD-A-208,748 proposes to notify the operator at the initiation of a tying cycle if a twine failure has occurred so that operation of the baler may be stopped before the bale is ejected. During normal operation, a sensor arm, disposed in the path of travel of the binding material and hence engaged thereby, is resiliently held in a position away from an electrical switch. When the binding material breaks, the sensor arm is released and the electrical switch is actuated whereby the operator is alerted to the twine failure. Although corrective action then can be taken while the broken bale is still in the bale chamber, it nevertheless will be appreciated that the foregoing inevitably leads to down time of the machine which is unacceptable.